Tìm x biết:
1) 1/4+3/4.x=5/2
2) |x-1/4|=3/8
3) (x-3/2): 3 1/2=4/7
4) 8^x : 4^x=4
5) |x+2/3|-5=-2
6)72-x/x+40=7/9
Tìm x biết:
1) 1/4+3/4.x=5/2
2) |x-1/4|=3/8
3) (x-3/2): 3 1/2=4/7
4) 8^x : 4^x=4
5) |x+2/3|-5=-2
6)72-x/x+40=7/9
Đáp án:
$\begin{array}{l}
1)\frac{1}{4} + \frac{3}{4}x = \frac{5}{2}\\
\Rightarrow \frac{{1 + 3x}}{4} = \frac{{10}}{4}\\
\Rightarrow 1 + 3x = 10\\
\Rightarrow 3x = 9\\
\Rightarrow x = 3\\
2)\\
\left| {x – \frac{1}{4}} \right| = \frac{3}{8}\\
\Rightarrow \left| {\frac{{8x}}{8} – \frac{2}{8}} \right| = \frac{3}{8}\\
\Rightarrow \left| {\frac{{8x – 2}}{8}} \right| = \frac{3}{8}\\
\Rightarrow \left| {8x – 2} \right| = 3\\
\Rightarrow \left[ \begin{array}{l}
8x – 2 = 3\\
8x – 2 = – 3
\end{array} \right. \Rightarrow \left[ \begin{array}{l}
8x = 5\\
8x = – 1
\end{array} \right.\\
\Rightarrow \left[ \begin{array}{l}
x = \frac{5}{8}\\
x = – \frac{1}{8}
\end{array} \right.\\
3)\\
\left( {x – \frac{3}{2}} \right):3\frac{1}{2} = \frac{4}{7}\\
\Rightarrow \left( {x – \frac{3}{2}} \right):\frac{7}{2} = \frac{4}{7}\\
\Rightarrow x – \frac{3}{2} = \frac{4}{7}.\frac{7}{2}\\
\Rightarrow x – \frac{3}{2} = 2\\
\Rightarrow x = 2 + \frac{3}{2}\\
\Rightarrow x = \frac{7}{2}\\
4)\\
{8^x}:{4^x} = 4\\
\Rightarrow {\left( {8:4} \right)^x} = 4\\
\Rightarrow {2^x} = 4\\
\Rightarrow {2^x} = {2^2}\\
\Rightarrow x = 2\\
5)\\
\left| {x + \frac{2}{3}} \right| – 5 = – 2\\
\Rightarrow \left| {x + \frac{2}{3}} \right| = 3\\
\Rightarrow \left[ \begin{array}{l}
x + \frac{2}{3} = 3\\
x + \frac{2}{3} = – 3
\end{array} \right.\\
\Rightarrow \left[ \begin{array}{l}
x = \frac{7}{3}\\
x = – \frac{{11}}{3}
\end{array} \right.\\
6)\\
\frac{{72 – x}}{{x + 40}} = \frac{7}{9}\left( {dkxd:x \ne – 40} \right)\\
\Rightarrow 9\left( {72 – x} \right) = 7\left( {x + 40} \right)\\
\Rightarrow 9.72 – 9x = 7x + 7.40\\
\Rightarrow 7x + 9x = 72.9 – 7.40\\
\Rightarrow 16x = 368\\
\Rightarrow x = 23
\end{array}$